Support HMAC-SHA256 signing method

[eborden]

HMAC-SHA256 is not technically required by the OAuth spec. However it is a very common signing method across many implementations.

The protocol defines three signature methods: HMAC-SHA1, RSA-SHA1, and PLAINTEXT, but Service Providers are free to implement and document their own methods.

https://oauth.net/core/1.0/#signing_process

This commit:

• Exposes HmacSha256 as a value in the SignatureMethod.
• Supports HmacSha256 within makeSignature via Crypto.Hash.SHA256.
• Encodes HmacSha256 as "HMAC-SHA256" via QueryValueLike.